Sieve array and precipitator device and method of treating exhaust

ABSTRACT

A vibrating wet precipitator is designed to remove particulates from particulate-laden hot gas. The precipitator includes an array of vertical wet cords stretched within a duct. The cords are tuned to vibrate due to the gas flow by controlling key parameters such as gas flow, velocity, cord length and diameter so that particulate collection and heat transfer efficiency are maximized. The cords are part of sieves. A plurality of these sieves are arranged to define a plurality of gaps, through which the exhaust flows. The sieves and thus the cords are space so that a vortex from a first cord affects an adjacent cord and subsequently cord. The particles are then absorbed in liquid, which can be passed through a heat exchanger filtered and subsequently reused. Preferably the cord arrangement is designed to allow the cords to vibrate at high frequencies, typically 10 to 100 Hz, to maximize particulate collection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry of International PatentApplication No. PCT/US2016/28606 filed on Apr. 21, 2016, which claimspriority to U.S. Provisional Patent Application No. 62/209,532 filed onAug. 25, 2015 and U.S. Provisional Patent Application No. 62/150,494filed on Apr. 21, 2015, the disclosures of which are expresslyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates generally to a precipitator device andmethod of treating an exhaust, and more particularly, to a plurality ofsieves for treating an exhaust.

BACKGROUND

Traditional precipitators, such as electrostatic precipitators, andscrubbers are widely used for treating exhaust containing gaseouspollutants and/or particulate emissions. For example, industrialprocesses, such as power and heat generation, may generateenvironmentally harmful particulate and gaseous emissions that mayremain suspended in the air. These emissions often present healthhazards when inhaled by humans and animals. Also, the particulateemissions tend to settle on equipment and buildings and may causediscoloration or even interfere with the proper function of theequipment. As such, it is important to remove these particulateemissions from the exhaust.

The exhaust can be passed through a traditional heat exchanger forrecovering thermal energy from the exhaust. After all, many industrialprocesses discharge exhaust into the environment at an elevatedtemperature and recovering this thermal energy provides for anopportunity to improve the efficiency of the industrial process.Industrial processes capable of discharging exhaust containing gaseouspollutants at an elevated temperature may also be fitted with a scrubberand/or a wet electrostatic precipitator (“wet ESP”) to both removegaseous pollutants, such as particulate emissions, and recover thermalenergy. Wet electrostatic precipitators typically include a liquid, suchas water, to capture both particulate and gaseous emissions as well asthermal energy, which may be directed through a heat exchanger forimproved efficiency.

While electrostatic precipitators, scrubbers, and heat exchangers aregenerally known for use with industrial processes, the effectiveness oftreating the exhaust has been limited, at least to some extent, bytraditional design limitations and the wide variety of differentcomponents necessary for treatment. For example, electrostaticprecipitators, scrubbers, and heat exchangers configured for treatingexhaust typically require unique alloys and coatings that increaseoverall cost and limit available space. Thus, the amount of surface areaavailable to any one of the precipitators, scrubbers, and heatexchangers is reduced and, similarly, reduces the effectiveness of thetreatment. In addition, traditional wet electrostatic precipitatorsoften produce a liquid mist that increases the likelihood ofelectrically shorting one or more electrodes, which also reduces itseffectiveness for collecting particulate emissions.

There is a need for a device and method of treating an exhaust thatimproves treatment effectiveness, reduces complexity, reduces costs, andaddresses present challenges and characteristics such as those discussedabove.

SUMMARY OF THE INVENTION

The objective of this invention is to use an array of wet vibratingcords (cylinders or ropes) to capture particulates from a hot gas streamalong with efficient energy recovery. When particulate-laden hot gasflows through an array of vertical wet cords, they tend to vibrate dueto vortex shedding. In particular, the cords will have vibrationsprimarily perpendicular to the flow direction and less pronounced onesin the flow direction. In this invention, these vibrations are tunedtowards a frequency band close to the natural frequency of the array ofcords so that the vibrational velocities and accelerations are enhanced.With increased vibrational velocity and acceleration, particulatecapture is increased with dramatic enhancement in energy and masstransfer. This array of wet vibrating cords thus functions veryefficiently as a particulate capture and energy recovery instrument.

The objective of this invention is to tune these vibrations in afrequency band as close as possible to the natural frequency of thearray of cords so that the vibrational velocities and accelerations areenhanced. Higher cord accelerations and higher vortex-sheddingfrequencies mean higher interacting forces between the gas flow and thecords. With increase in vibrational velocity and acceleration, there isenhancement in particulate capture. In addition, there is increase inheat transfer and dramatic increase in transfer of the water vapor andwet liquid droplets between the wet cords and the flowing gases fromwhich most of needed water is extracted. The particle size is increasedwhen the gases are supersaturated by cooling down and condensationoccurs on the particles. There is also addition of droplets to the highvelocity gas stream by shearing off from the water film on the cords.Therefore, this array of wet vibrating cords can be used to perform thefollowing functions:

-   -   (A) as a particulate capture instrument due to (I) high        impaction efficiency (I) particle growth, (ii) increase in        turbulence shear (iii) generation of water droplets that act as        a scrubber for the particles, and (iv) increased evaporation and        condensation of water on the particles.    -   (B) as an energy recovery instrument due to (i) high heat        transfer (ii) increase in turbulence shear (III) generation of        water droplets that act as a scrubber, and (iv)increased        evaporation and condensation of water on the cords.    -   (C) as a scrubber for gases and vapors due to the scrubbing        action of the water film on the cords and the wet particulates,        and the scrubbing by the water droplets generated by shearing        action of the high velocity gases on the vibrating cords.

This technology can be combined with any precipitation setup usingelectrical charging of particulates and installed downstream. But bothunits need to be fine-tuned to achieve the best result. If notfine-tuned, the cords will still vibrate and will still contribute tothe particulate capture to a certain degree, but not nearly as much asif tuned; this is the key feature of this invention.

Since this technology is inexpensive and has large particulatecollection efficiency even without using electric power, i.e. itsapplication is simple and safe, it could find wide residential andcommercial uses. The following sections provide details on the effect ofvibrations on particulate capture, heat transfer, energy recovery andparticle growth.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description given below serve to explain the invention.

FIG. 1A is a perspective view of a first exemplary embodiment of adevice for treating an exhaust having a plurality of sieve assemblies.

FIG. 1B is an overhead plan view of the apparatus of FIG. 1A in a duct.

FIG. 2 is an overhead plan view of a second exemplary embodiment of adevice for treating an exhaust having an array of sieves.

FIG. 3A is a top sectional view of an exhaust flowing along a sieve ofthe sieve assembly shown in FIG. 1A with the sieve in an initialtransverse position.

FIG. 3B is a top sectional view of the exhaust and the sieve similar toFIG. 3B, but showing the sieve in a terminal transverse position.

FIG. 4A is a diagrammatic depiction of a stationary string and avibrating string showing the straight line motion of the vorticesshedding off the stationary cylinder.

FIG. 4B is a diagrammatic depiction of a plurality of vibrating cordsshowing the cylinder vibrating in one direction with the vortex swayedin the opposite direction; in the next vibrating cycle, the motiondirections of the cylinder and the vortex are changed

FIG. 4C is a diagrammatic depiction showing the capture of big particlesby impaction and the capture of small particles flowing close to thevibrating cylinders and taken by the swayed vortices and captured on theback of wet cylinders.

FIGS. 5A and 5B are diagrammatic depictions of a vibrating string.

FIG. 6 is a side cross-sectional view of an alternative embodiment ofthe present invention.

FIG. 7 is an overhead cross-sectional view of the embodiment shown inFIG. 6.

DETAILED DESCRIPTION

In the vibrating wet precipitator 10 (VWP) shown in FIGS. 1A and 1B,particulate-laden hot gas flows through an array 30 of vertical wetcords 26 stretched within duct 12. The cords 26 are tuned to vibrate dueto the gas flow. In particular, the cords 26 will have vibrationsperpendicular to the flow direction and to a minor degree in the flowdirection too. By controlling the key parameters, such as gas flowvelocity, cord length and diameter and its physical properties, as wellas cords' spacing, amount of cleaning water, particle collection andheat transfer efficiency are maximized.

FIGS. 1A and 1B shows an exemplary embodiment of precipitator device 10with the plurality of cords 26 fixed at top and bottom with a desiredtension. There are three rows of cords labeled 26(a), 26(b) and 26(c)which are part of sieves 38, 40 and 42, respectively. As discussedbriefly above, the plurality of sieves 24 are arranged to define theplurality of gaps 28 through which the exhaust flows from a duct inlet20 to a duct outlet 22 with like numbers indicating like featuresdiscussed above. According to the exemplary embodiment, the elongatedcord 26 is in the form of a rope having primarily but not necessarilycircular cross section. However, it will be appreciated that alternativematerials and structures that vibrate may be so used.

The sieve assembly 16 includes first, second, and third sievearrangements 38, 40, 42, respectively. Each of the sieve arrangements38, 40, 42, includes sieves 24 offset and parallel from each other alonga linear row. The plurality of sieves 24 are oriented generallyvertically/longitudinally and, as such, perpendicular to the transverseflow direction of the exhaust. While the sieves 24 are distributed aboutthe flow chamber 14 generally evenly to define like gaps 28, it will beappreciated that more or less sieves 24 may be used with varyingorientation and placement within the duct 12.

Preferably, the sieves and thus the ropes 26(a) through 26(c) are spacedso that a vortex from a first rope 26(a) affects adjacent rope 26(b) andsubsequently rope 26(c) etc. as shown in FIG. 4B.

Each sieve arrangement 38, 40, 42 includes the generally horizontallyextending support member 44, which defines a liquid supply conduit 46extending therethrough. The support member 44 supports the generallyvertical orientation of the elongated cords 26 and is configured tointroduce the liquid therein to define a sieve inlet 50. According tothe exemplary embodiment, the support member 44 and the liquid supplyconduit 46 are in the collective form of a single elongate tube;however, it will be appreciated that many other designs for supportingthe sieves 24 and providing for the supply of liquid to the sieve inlet50 may be so used.

According to the exemplary embodiment, the sieve inlet 50 and a sieveoutlet 54 are positioned at opposing end portions of the elongated cords26. However, it will be appreciated that the sieve inlet and outlet 50,54 may alternatively include or additionally include further structures,which may define, respectively, the inlet and outlet.

The vibrating wet precipitator device 10 further includes a liquidcollector 56 positioned proximate to the sieve outlets 54 for collectingthe liquid being discharged from the sieve outlets 54. In addition, theelongated cords 26 connect to the liquid collector 56 as another framemember 56. As such, the elongated cords 26 extend between the framemembers 44, 56 and are configured to vibrate therebetween.

The elongated cords 26 are more particularly configured to vibrate bypassing the flow of the exhaust transversely therealong. Alternatively,the elongated cords 26 may be operatively connected to a vibrationmechanism configured to actively vibrate the elongated cords 26 duringuse.

The liquid collector 56 is in the form of a tray 56 that includes abottom 58 and surrounding sidewalls 60 configured to guide the liquid toa liquid treatment system 62. The liquid treatment system 62 includes apump 64, a filtration system 66, and a heat exchanger 68. The pump 64 isconfigured to direct the liquid from the liquid collector 56 to thefiltration system 66, which is configured to remove particulateemissions from the liquid. The pump 64 then continues to direct theliquid through the heat exchanger 68 for recovering thermal energy fromthe liquid. While the liquid may be removed from the precipitator device10, the liquid may also be redirected back into the liquid supplyconduit 46 for reuse through the elongated cords 26, as illustratedschematically in FIG. 1. It will be appreciated that the pump 64,filtration system 66, and heat exchanger 68 may be selected andassembled in order to accommodate any performance requirements fortreating the exhaust of any given industrial process. For this reason,the pump 64, filtration system 66, and heat exchanger 68 may be selectedand assembled per known requirements readily appreciated by those havingordinary skill in the art.

FIG. 1B shows an exemplary embodiment of a vibrating wet precipitatorhaving three arrays 30, 32 and 34 of cords 26. The number of arrays, aswell as the number of cords per array, can be modified to maximizecollection efficiency.

In use, as shown in FIGS. 3A-3B and FIG. 4B, the flow of the exhaustforces the elongated cord 26(a) to vibrate between the first and secondpositions. Relatively large particulates 52 impact the elongated cords26(a), which collect these particulates thereon. However, the relativelysmall particulates 53 tend to follow the flow of the exhaust around thevibrating elongated cords 26(a). To this end, the elongated cords 26(a)generate the trailing vortices and effectively capture and collect thesmaller particulates within each trailing vortex. The vortices fromcords 26(a) will impact cords 26(b) which, in turn impact cords 26(c),all of which will be vibrating as shown by FIGS. 3A and 3B. Therelatively small particulates impact with each other and coalesce intolarger particulates that also impact the elongated cords 26(a)-26(c) forremoval. The beneficial flow turbulence is further enhanced bysmaller-amplitude vibrations of the ropes in the flow direction; thesevibrations have two times higher frequencies than those in the lateraldirection. The liquid flowing along the elongated cords 26 may thengather the particulates and direct the particulates to the tray 58 forremoval by the liquid treatment system 62.

Contrary to conventional electrostatic precipitators, particulatecapture efficiency is expected to be higher at higher gas velocities andaccompanying high vibrational velocities and accelerations of the cords,caused by higher vortex-shedding frequencies, and particles are morelikely to deposit by impaction at high relative velocities andacceleration. Hence dimensions of the vibrating precipitator could bedecreased and the cords could be mounted even in the existing ducts,instead of expanding ducts to reduce flow velocities, like inconventional electrostatic precipitators.

Increasing the size of the particles is very effective means forimproving the collection by impaction. Particle growth can occur bycollision and coagulation or by condensation of water from the gasstream. The turbulent shear due to the high flow velocity and theperturbations by the cords which vibrate and detach vortices with veryhigh accelerations will produce high collision rates due to high forcesexerted by the ropes on the water flowing down them, so that particlegrowth rates become significant feature of the vibrating precipitator.

Small size particles are difficult to capture by impaction even thoughthe velocity in the vibrating collector is high. However, they can becaptured by small droplets of water that are generated by vibratingcords in the high velocity gas. It is known that the collectionefficiency of small droplets of water have high efficiency forcollecting particles from a flow of gases. It should be noted that spraychambers are used for particulate removal (scrubbing). The taut cordsvibrating at very high acceleration exert large forces on water filmflowing down them and generate a spray chamber environment similar tothat in scrubbers and suitable for capture of very fine particulates.

Effect of Cord Vibration on Particulate Capture:

Although cylindrical cords tend to vibrate in a cross flow with a verysmall amplitude, the present invention utilizes high frequencyvibrations, typically with frequencies in the range of 10-100 Hertz. Asa consequence, vibration velocities and accelerations of cords are veryhigh. Fundamental transverse vibrations are especially pronounced whenvortex shedding frequency locks onto the cord natural frequency (seeFIGS. 3A and 3B). It happens over the range 4<v/(f_(n)D)<8 where, v isgas velocity, D is the cord diameter and f_(n) cord's natural frequency,which depends on cord tension and its length and unit mass. As explainedbelow, smaller vibration also occurs out of that range. Given gas flowconditions, there is a lot of room for cords' tuning to produce desiredvibration amplitudes and frequencies buy changing their length, tensionforce, specific weight, including changes in the cross section shape.Laboratory tests show that the particulate collection efficiency issignificantly reduced if the cords are not adjusted and tuned tospecific flow and dust loading parameters.

Together with higher vortex-shedding frequencies produced at elevatedflow speeds, these high vibration frequencies will produce additionalturbulence that breaks up the boundary layer on the wet cords, andincrease both diffusional deposition of particles as well as impactionsdue to the high vibrational velocity/acceleration of the cords. Theimpaction process is determined by the relative velocity between theparticle and the cord. The vibrational velocity and the flow velocitytogether produce a higher effective velocity Stokes number which resultsin higher rates of impaction as explained below. This can increaseparticulate capture with or without need of charging it, leading toeither significant reduction of the electrical power or even itscomplete elimination, i.e. exclusion of transformer/rectifier units,discharge electrodes, and the corresponding control equipment,maintenance etc.

Migration velocity is the key parameter on which ESP efficiency depends.In conventional ESPs it is very small, typically only 0.02-0.08 m/s.depending on dust properties in various industries. They are the maincause of low ESP efficiencies. In the present invention, thesevelocities are increased, not by moving particles faster towards the wetcords but because the cords are moved fast towards the particles. Andfor producing such a speed, charging particulate would help but it isnot a requirement.

To increase particulate capture efficiency, cords are kept wet by asupply of fluid (typically water but not excluding other media) at thetop. Cords can be made from a number of materials that could be bothhydrophilic and hydrophobic, because continuous cord vibrations willsmooth the fluid flow along them. In absence of a high-voltage field,water droplets can be allowed to shear off without creating shortcircuits like in conventional electrostatic precipitators, thusproducing better particulate capture by scrubbing action. Thus, when nocharging electrodes are used, hydrophobic cords can be employed. Whereashydrophilic cords are preferred with an ESP, i.e. when the particulateis charged.

In case of a bundle or array of cords (FIG. 1) with several rows ofcords being in the wake of each other these transport processes are morecomplicated and depend on longitudinal and transfer pitches. Thecollection efficiency of an array of cords of diameter D with alongitudinal pitch of S_(L) in the flow direction, and a transversepitch of S_(T) can be calculated as:

${{Eff}\left( {N\mspace{14mu}{rows}} \right)} = {1 - \left\lbrack {1 - \frac{\eta}{S_{Tn} - \frac{\pi}{4S_{L\; n}}}} \right\rbrack^{N}}$

Where N=number of rows, S_(Tn)=S_(T)/D, S_(Ln)=S_(L)/D and the singlecord (cylinder) efficiency (η) is based on the transport process (e.g.,impaction, diffusion, etc.). The array can be designed on the basis ofthe above equation for optimum particulate transfer.

Heat Transfer

Heat transfer from the gas to the water flowing down the cords issignificantly increased by cords' vibration; this heat flux is enlargedby increasing both vibration amplitudes and frequencies as well as bydecreasing the cord diameter. Since geometry of cords (length, diameter,cross section shape) and their physical properties (material used,tension applied) can be changed/adjusted, so will be this amount, aswell as the amount of dust collected. It is known that at each point onthe cord surface the heat transfer depends on local temperature and thetemperature and velocity of the boundary layer. The total heat flux isproportional to the total surface area of the cord and the local heattransfer coefficient. Heat transfer is best enhanced if the cord isoscillating near the Strouhal frequency.

High heat transfer coefficients are particularly present athigh-amplitude and high-frequency vibration and especially at thetrailing half of the cord where the influence of the vortex rollupprocess is most pronounced This corresponds to regimes in which v/f D (fis cord's frequency in Hz) and for the amplitude/diameter ratio A/D<0.8.In this regime vortices formed behind the cords have short formationlength (they form very near cord surface) and are therefore effective inheat and mass transfer.

In case of a bundle of cords with several rows of cords being in thewake of each other, there are several studies that provide the heattransfer to an array of stationary cylinders. The effect of cordvibrations is to produce vortices closer to the cylinder and heattransfer is enhanced if the vortex is swayed by vibrations close to thecylinder (FIG. 4A). The heat transfer can now be optimized by adjustingthe pitch of the cylinders and the vortex formation length so that thevortex affects multiple adjacent cylinders (FIG. 4B).

Water Vapor Transport

An important effect of wet cord vibration is the tremendous increase inthe transport of vapor from the cords. The ratio of the mass transfercoefficient from a vibrating wet cylinder to stationary wet cylinder isgiven by:

$\frac{k_{V}}{k_{st}} = {0.117\left\lbrack {4{{{Af}\left( {{2\; A} + D} \right)}/v}} \right\rbrack}^{0.85}$

Where k_(v) is the mass transfer coefficient for the vibrating cord andk_(st) is the mass transfer coefficient for the stationary cord, A isthe amplitude, D is the diameter and v is the kinematic viscosity. Thisequation was proposed by R. Lemlich and M. R. Levy (AlChE Journal, Vol7, p. 240, 1951) based on their experiments in which they were able toget 660% increase in mass transfer.

Therefore, the effect of the wet cord vibration will be to immediatelysaturate the gas stream when it enters the cord array. Simultaneously,the heat transfer process will cool down the gases and the flow willbecome supersaturated with water vapor which will condense on theparticles surfaces and the wet cords. It can be shown the condensationalgrowth of particles will occur very fast, and hence increase theircapture by impaction as described below.

Generation of Water Droplets

The combination of high vibration frequency, velocity and especiallyhigh accelerations of the wet cords and high velocity of the gas streamcan be expected to generate droplets that shear off from the water filmon the cords. The size of the droplets can be adjusted by the surfacetexture of the cord and their size will also depend on the shear forceson the water film, and in the vortices induced by the flow. Thesedroplets are very efficient in capturing fine particulates by Brownianmotion and by turbulent shear.

Therefore, the vibrating precipitator can function as a spray chamber tocapture particulates from the gas stream.

In order for the evaporation and condensation processes to occur withina system, the design would include an initial array of wet cords withsufficient water supply that can saturate the gas by vapor transport. Inthis section, the biggest particles will be captured by impaction.

There would then be a middle array where the gases would have cooleddown sufficiently to start the condensation on the particles and the wetcords. In this section the smaller particles will grow, and canparticipate in scrubbing of toxic vapors or gases. The cross sectionalarea of the VWP can be increased in this section to increase theresidence time for condensation and scrubbing.

In the final section the particles would be captured by impaction on thearray of wet cords from a high velocity gas stream. More condensation ofwater will occur; and the total condensation will contribute to the heattransfer from the hot gases. It should be noted that the wet stringswill act as a scrubber throughout all the sections.

Flow-Induced Vibration

With reference to FIGS. 5A and 5B, for elastically supported string/ropeof length L vibrating in the y-direction perpendicular to the gas/fluidflow, displacement is described by the differential equation (R. D.Blevins: Flow-Induced Vibration, Van Nostrand Reinhold, New York, 1990):mÿ+2mξω _(n) {dot over (y)}+ky=½ρ_(G)ν² DC _(L) sin ω_(ν) t  (1)whose solution is:y(t)=A cos(ω_(ν) t−ϕ)  (2)where amplitude A is

$\begin{matrix}{{A = \frac{\frac{1}{2}\rho_{G}v^{2}D\; C_{L}}{{k\left\lbrack {\left( {1 - \left( \frac{\omega_{v}}{\omega_{n}} \right)^{2}} \right)^{2} + \left( {2\xi\frac{\omega_{v}}{\omega_{n}}} \right)^{2}} \right\rbrack}^{1/2}}},m} & (3)\end{matrix}$and other quantities are:

-   -   ÿ, {dot over (y)}—acceleration (m/s²)and velocity (m/s)    -   ρ_(G)—gas density, kg/m³    -   ξ—structural damping; for many objects is 1%-3%, i.e.ξ≅0.01-0.03        and it can be easily experimentally determined    -   ν—gas velocity, m/s    -   D—string diameter, m    -   C_(L)-dimensionless lift coefficient; for circular cylinders in        a wide range of Reynolds numbers C_(L)≅1    -   ω—natural frequency of the string, 1/s

$\begin{matrix}{\omega_{n} = {{\frac{\pi}{L}\sqrt{\frac{T}{m}}} = \sqrt{\frac{k}{m}}}} & (4)\end{matrix}$

-   -   L—length of string, m    -   T—tension in the string, N    -   m—unit mass of the string, kg/m    -   k—stiffness ϕ-phase    -   ω_(v)—vortex shedding frequency, 1/s

$\begin{matrix}{\omega_{v} = {2\pi\frac{Sv}{D}}} & (5)\end{matrix}$

-   -   S—dimensionless Strouhal number; for circular cylinders S≅0.2

In case of flow between a set of parallel cylinders, the Strouhal numberin (5) is based on the largest average velocity between the cylinders.If that opening is h, using conservation of mass:

$\begin{matrix}{\omega_{v} = {2\pi\frac{Sv}{D}\frac{D + h}{h}}} & (6)\end{matrix}$

From (2), vibration velocity is

$\begin{matrix}{v = {\frac{dy}{dt} = {{- A}\;\omega_{v}{\sin\left( {{\omega_{v}t} - \phi} \right)}}}} & (7)\end{matrix}$such that the maximum velocity and the maximum acceleration in thedirection perpendicular to the gas flow areν_(max) Aω _(ν) ,m/s α _(max) Aω _(ν) ² ,m/s ²  (8)

Velocity is therefore proportional to the vortex shedding frequency,while acceleration can be very large because it is proportional to thesquare of that, otherwise high frequency. Natural frequency of the cordis of the order of 10{circumflex over ( )}2 rad/sec, while thevortex-shedding frequency is several times higher. Given gas flowconditions, gas flow speed in particular, Eq. (4) explains what needs tobe done to increase the natural frequency of the cords towardsvortex-shedding frequency. At lower gas velocities, in order to increasethe natural frequency, the effective length of the cords can be reducedby a) restraining cords motion not only at its ends but also at one ormore locations in between, or b) by suspending several shorter cords(with their own water supply systems) instead of using a single longcord.

For cords suspended at the ends only, Eq.s (7), (8) refer to the middlepoint of the vibrating string x=L/2, while at any other point

${{y\left( {x,t} \right)} = {A\;{\sin\left( {\frac{\pi}{L}x} \right)}{\cos\left( {{\omega_{v}t} - \phi} \right)}}},{0 \leq x \leq L}$

Consequentially, the largest displacements and velocities are near themiddle of the string and are gradually decreasing towards the two ends.

It is reasonable to assume that migration velocity of a particle passingbetween vibrating cylinders (which vibrate in a direction roughlyperpendicular to the particle's path) on which it needs to be depositedis proportional to the velocity (7). Due to large inertia, biggerparticles will be less swayed by the flow (displaced by movingcylinders) while small particles will adjust to the flow.

Typical values of cord diameters are 1-5 mm, tension force 100-1000 N,cord lengths 0.5-3 m, the wet-cord weight about 5-100 grams, and gasvelocities ranging 3 to 30 m/s.

Particle Growth by Collision, Coagulation and Condensation

Particulate matter suspended in gases can grow in size by several means.For the Vibrating Precipitator, two modes can be expected to dominate:collision/coagulation and condensation.

Collision and coagulation occurs when particles come in contact witheach other and agglomerate. In analytical treatments of conventionalESPs, it is assumed that the particles in the suspension aresufficiently separated so that their interaction can be neglected. Theprimary reason for this is that the viscous forces of the gases betweenthe particles prevent particles from colliding like gas molecules, andthe viscous force must be overcome by the van der Waals forces forcollisions to take place (Ref: M. K. Alam, “The Effect of Van Der Waalsand Viscous Forces on Aerosol Coagulation,” Journal of Aerosol Scienceand Technology, Vol. 6, pp. 41-52, 1987).

The rate of particle collisions in a gas medium has been analyzed bymany researchers, and it has been shown that the collisions can takeplace due to shear in the fluid flow. The significance of collision andcoagulation in the gas stream can be evaluated by comparing thecharacteristic time (or time constant) for coagulation, which is givenby (ref: S. K. Friedlander, “Smoke, Dust and Haze, Fundamentals ofAerosol Behavior, Wiley, New York, 1977).

${\tau = {\frac{2}{{KN}_{d}} = {3{\mu/\left( {4{kTN}_{d}} \right)}}}},$where μ is the dynamic viscosity, K is the collision rate, k is theBoltzmann's constant, T is the temperature and N_(d) is the numberconcentration of particles.

For turbulent shear, the collision rate and growth is controlled byturbulent energy dissipation which has been measured in the wake ofcylinders (Ref: X. Zhang, W. Zhong, J. Yang and M. Lou, “DimensionalAnalysis and Dissipation Rate Estimation in the Near Wake of a CircularCylinder, Journal of Applied Mathematics and Physics, Vol 2, pp 431-436,2014). The characteristic time is much less than 1 second in the vortexwhen gas flow rate is about 16 m/sec with typical particulateconcentration of 3 g/m3 in the ESP. Therefore, turbulent shear willcontribute to particle growth.

Condensation will take place on the particles when the gas streambecomes supersaturated. In the array of wet cords this will happen veryquickly because the vibrating cords will produce moisture at a high rateif the gas is under-saturated. There are also high heat transfer ratesbecause of the vortex shedding by the vibrating cords which cools thegases. As the particle-laden gases move through the array, they willgrow by condensation according to the equation (see Friedlander):

$\frac{{dd}_{p}}{dt} = \frac{4{{Dv}_{m}\left( {P_{g} - P_{d}} \right)}}{{kT}\; d_{p}^{2}}$

Where d_(p) is the particle diameter, D is the diffusion coefficient,ν_(m) is molecular volume of water, (P_(g)−P_(d)) is the water vaporpressure difference between the gas stream and the particle surface. Fora typical combustion flue gas cooling down to about 40° C. with approx.15% water vapor, the growth time is of the order of milliseconds forparticles smaller than 5 microns. From the above equation, it is alsoobvious that smaller particles will grow even faster. Therefore,condensational growth will greatly increase the particle size in thevibrating wet precipitator.

Particulate Capture by Diffusion, Interception and Impaction

Particulate capture is the goal the vibrating wet precipitator 10.Without vibration, the capture becomes significant only for biggerparticles (bigger than 1 micron). The vibration of the wet cords changesthe gas flow patterns significantly—the turbulence intensity isincreased around the vibrating string, while the relative velocitybetween the string and the particles also changes. These phenomenaproduces enhancement in particulate capture through the followingmechanisms:

-   -   (i) Convective Diffusion: Since the flow is expected to be        turbulent, the collection efficiency is a function of the        Reynolds number and the Schmidt numbers. Research has shown that        the particulate capture by filter fibers is enhanced by the        vibration Reynolds number (calculated on the basis of the        vibrational peak velocity) which leads to increased diffusion        intensity around the cylindrical fibers. For a vibrating string        with frequency f (Hz) and amplitude A, the vibrational Reynolds        number (Re_(v)) and the vibrational velocity (maximum) is given        by:

${{Re}_{v} = \frac{U_{v}D}{v}},$

-   -    and Vibrational velocity V_(ν)=2πfA        -   Research by Kim et al. (Ref: S. C. Kim, H. Wang, M. Imagawa            and D. Chen, “Experimental and Modeling studies of the            Stream-wise Filter Vibration effect on the Filtration            Efficiency”, Aerosol Sci. and Tech., 40:389-395, 2006) on            fibrous filters demonstrated up to 60% increase in capture            efficiency with velocity as small as V_(ν)=0.03 m/s The            vibrational velocity for the Vibrating Precipitator can be            quite high; for an amplitude of 1 mm and a frequency of            2000, this value is V_(ν)=26 m/s. Therefore, high capture            efficiency by convective diffusion can be expected.        -   (ii) Impaction: Particulate capture by impaction in            turbulent flow is primarily determined by Stokes number            (Stk), defined by

${Stk} = \frac{\rho_{p}d_{p}^{2}C_{c}U}{9\mu\; D}$

Where U is the gas flow velocity, D is the diameter of the string, isthe kinematic viscosity of the gas, and C_(c) is a correction factor(Ref: P. Douglas and S. Ilias, “On the Deposition of Aerosol Particleson Cylinders in Turbulent Cross Flow”, J. Aerosol Science, Vol 19 (4):451-461, 1988). The efficiency of capture goes up rapidly with Stokesnumber. In a vibrating array of cords, the Stokes numbers increases bythe following mechanisms:

-   -   (a) The velocity U is now the resultant velocity of the gas        stream and the vibrational velocity. Therefore, the particles        are much more likely to impact the string. Another way of        looking at the mechanism is that the vibration increases the        interception area since the particle moves perpendicular to the        vibrational velocity.    -   (b) The Stokes number is also increased by particle size growth        by coagulation due to the turbulent shear in the vortex behind        each string, and    -   (c) The condensation of supersaturated gas stream as it moves        through the array of wet cords increases the particle size and        thereby its impaction efficiency.

EXAMPLE

A bench-scale test unit consisting of a 12 foot long inlet, 4 footoutlet and 2 foot long test section between the two. The test sectionhouses two or eight 1-inch thick sieves, set three inches apart. Eachsieve consists of 30 polypropylene 5-mm ropes (actually it is a singlerope, which is looped through holes) distanced 10 mm center to center,occupying 30×12-inch space with the total area of 360 square inches=0.23square meters. Ropes in the neighboring sieves were aligned, notstaggered,

In order to be able to apply variable tension force to the rope(s), ontop a single rope is looped through holes in a thick hollow beam whichcould move up or down. Tension in the rope(s) was 25, 35 or 45 pounds.

PVC pipes were used to deliver water running down the strings. Theamount of water used in all tests was 0.75 liters per minute per cell,in all cells.

The 3-micron fly ash with concentration ranging from 30 to 70mg/m{circumflex over ( )}3 was injected into air at the inlet usingSCHENK AccuRate type MOD102M dust feeder.

All tests were conducted at room temperatures with air velocity of 25ft/s at inlet. The air/gas was drawn by the outside fan.

Fly ash collection efficiency was measured using two Thermo ScientificMIE ADS-1500 particle monitoring (PM) units and taking dust samples atcross sections 20 inches before the first cell and 20 inches after thelast cell. At each of the two cross sections, dust samples were taken atthree points: one located on the crossing of the central vertical andhorizontal duct's symmetry planes and the other two on the horizontalcentral plane left and right of that point. Then the readings of the PMunits were averaged.

The following table has dust collection efficiency results with 2 and 8cells, depending on tension force in the ropes.

Tension (lbs) 2-cell efficiency (%) 8-cell efficiency (%) 25 7.55 48.8635 21.45 60.86 45 7.135 64.18

The following two tables has pressure drop and heat transfer coefficientresults calculated following the algorithm from “Fundamentals of HeatExchanger Design” by Ramesh Shah and Dusan Sekulic, 2003. They werecalculated for non-vibrating, stationary cylinders at different airvelocities, assuming that the ropes are in staggered position and thatthe unit operated with air and water temps of 130 F and 80 F. The ropes'diameter is assumed to be 3 mm or 5 mm and the corresponding spacingbetween both the ropes in a single row and the rows themselves are 7 mmand 10 mm respectively. It should be noted that increases in heat andmass transfer due to relative movement between a fluid and a surface arealso accompanied by additional pressure drop in the flow. The firsttable gives the pressure drop results in inches of water at differentvelocities with 3 mm ropes. The second table gives the pressure drop andconvective heat transfer coefficient (h) for 3 mm and 5 mm ropes at airvelocity of 25 ft/sec.

Pressure drop in inches of water at different velocities #Rows (Cells)of 25 ft/s = 30 ft/s = 35 ft/s = ropes (3 mm dia) 7.62 m/s 9.14 m/s10.67 m/s 3 0.47 0.65 0.87 6 0.93 1.30 1.73 8 1.24 1.74 2.31

Calculations with center-to-center distance between rows of 10 mm, Vel =7.62 m/s (25 ft/s) Results for different diameter ropes 3 rows 5 rows 10rows Rope Tot h Tot h Tot h Spacing Area PressDrop W/m2K Area PressDropW/m2K Area PressDrop W/m2K 3 mm rope 0.007 4.25 0.47 256 7.09 0.78 27514.17 1.55 289 5 mm rope 0.010 4.96 0.60 243 8.27 1.00 261 16.53 2.00274±

Setting the ropes in staggered position will result in increasedparticulate collection efficiency at the expense of somewhat increasedpressure drop. Since heat and mass transfer are enhanced by increasedgas velocity and by reducing the ropes' diameter, using 3-mm ropesinstead of 5-mm, staggered in eight cells, the dust collectionefficiency is expected to be significantly higher than 64% obtained inthe test just described.

This system can be compared with a conventional ESP. A hypotheticalconventional horizontal-flow wet ESP with the rectangular inlet whosecross section is the same as in the test unit described above, i.e. itis 360 square inches (0.23 square meters). Further, assume the ESPoperates at gas temperature of 140 F, with water temperature of 80 F.Further, assume that the ESP is formed by three 1.5 meters tall and 1.7meters long plates, with the distance between the plates of 0.3 meters,with discharge electrodes in the middle. At the air speed of 25ft/s=7.62 m/s in the inlet, i.e. of 6.36 ft/s=1.94 m/s in the ESP, theflow rate is Q=3708 acfm=1.75 m{circumflex over ( )}3/s. Assume furtherthat the ESP operates in a utility collecting fly ash, and that the flyash migration velocity is very high, w=0.18 m/s (its typical value is0.03-0.20 m/s). Using the Deutsch-Anderson equation η=1−exp(−wA/Q), onefinds that the conventional ESP unit just described has the sameefficiency η=0.64 as the 30 times smaller in volume 30×12×8 inch testunit mounted in its duct, operating at room temperature, i.e. under muchless desired conditions, and without charging the particulate.

Although the present invention functions without charging the particles,in certain instances, the present invention can be incorporated into anelectrostatic precipitator. Unlike the embodiment shown in FIGS. 1A and1B, splashing liquid is undesirable in an ESP, thus, the gas velocity islowered.

Further, the cords are formed from a hydrophilic material such as ahydrophilic polymer. Tuning of the cords at lower gas velocities isaccomplished by reducing the cords' effective length by having severalshorter cords or by restricting motion of a single rope, not only at thetop and bottom, but also at one or more places in between. FIG. 2 showsan electrostatic precipitator which incorporates multiple precipitators.Precipitator device 110 includes a duct 12 having a flow chamber 14 anda sieve assembly 16. The sieve assembly 16 is positioned within the flowchamber 14 to define a flow path 18 therethrough. As shown in FIG. 2,the sieve assembly 16 can be arranged within the flow chamber 14 tocoordinate a three stage method of treating an exhaust flowing throughthe duct 12 from a duct inlet 20 toward a duct outlet 22. Thisembodiment utilizes electrodes 36 to charge the particles to facilitateparticulate capture. However, the electrodes are optional and notrequired in most applications.

According to the exemplary embodiment, the exhaust has excess thermalenergy and a plurality of particulate and gaseous emissions, both ofwhich may be removed and recovered from the exhaust during treatment.The sieve assembly 16 includes a plurality of sieves 24. As with thestructure shown in FIG. 1A, each of the sieves 24 includes an elongatedcord or cords 26 that partially obstructs the exhaust flowing along theflow path 18 and extends longitudinally therealong. The plurality ofsieves 24 define a plurality of gaps 28 therebetween for receiving theexhaust flowing from the duct inlet 20 to the duct outlet 22. Each ofthe elongated cords 26 is configured to vibrate between a first positionand a second position in order to generate a trailing vortex within theflow of the exhaust. As shown in FIG. 4A, the trailing vortex collectsrelatively small particulates flowing with the exhaust such that theparticulates impact with each other to form relatively largeparticulates. In turn, the particulates coalesce into largerparticulates that impact the elongated cords 26 for removal from theflow of exhaust. More particularly, the spacing of the cords or ropesare designed so that the vortex of a first or forward cord affects anadjacent cord as shown in FIG. 4B.

Furthermore, the sieves 24 are configured to receive a liquid, such aswater or an alkali solution so that the liquid flows, by gravity and/orcapillary action, along the elongated cords 26. More particularly theelongated cords 26 are formed from a liquid permeable material.—Thereby,the plurality of particulate and gaseous emissions (e.g., NO_(x),SO_(x), CO₂, and Mercury) and excess thermal energy passing through theduct 12 accumulates within the liquid for treating the exhaust, whichmay then be discharged to the environment. According to the exemplaryembodiment, the plurality of sieves 24 recovers particulate emissions,gaseous emissions, and thermal energy from the exhaust. However, it willbe appreciated that any number of sieves 24 may be used in any number ofarrangements and dedicated to scrubbing and/or recovery and removal ofeither one or both of the emissions or thermal energy. As such, the term“treatment” is not intended to limit the invention described herein.

A first stage of treatment includes a first portion 70 of the sieveassembly 16 positioned proximate to the duct inlet 20. As such, thefirst stage of treatment is upstream of a second stage and a third stageof treatment, which includes second and third portions 72, 74 of thesieve assembly 16, respectively. The first stage of treatment includesthe first portion 70 of the sieve assembly 16 configured to remove theplurality of particulate emissions from the exhaust via impaction andact as a scrubber, while also removing thermal energy from the exhaust.In contrast, the second stage of treatment includes the second portion72 of the sieve assembly 16 which is electrically grounded and aplurality of discharge electrodes 36 positioned proximate to the sieveassembly 16. The plurality of discharge electrodes 36 is configured tocharge particulate emissions within the exhaust. In turn, the secondportion 72 of the sieve assembly 16 attracts the charged particulateemissions, which then accumulate thereon for removal from the exhaust.Finally, in the third stage of treatment, the third portion 74 of theplurality of sieves 24 repeats the first stage of treatment for a finalrecovery of particulate emissions and thermal energy. Any liquid andcondensate that may form on the sieve assembly 16 may be recycled andreused for future treatment of additional exhaust as discussed below ingreater detail.

With respect to the plurality of discharge electrodes 36, it will beappreciated that the particulate emissions are generally given anegative electrical charge by passing these particulate emissionsthrough a region in which gaseous ions flow (i.e., a corona). Morespecifically, an electrical field forms between the discharge electrodes36 and the grounded elongated cords 26, which is conductive due to theliquid flowing therealong. Each of the discharge electrodes 36 isoperatively connected to an electrical current supply in order tomaintain a high voltage between the discharge electrodes 36 and theelongated cords 26, which act as a collection electrode. Thus, it willbe appreciated that the precipitator device further includes electricalequipment as well, for generating a high-voltage supply, such as ahigh-voltage transformer and a rectifier. These and other components maybe operatively connected to the discharge electrodes 36 and elongatedcords 26 as is presently understood in the state of the art.Alternatively, each of the sieves 24 may further include a collectionelectrode, such as a frame member positioned proximate to the elongatedcords 26, which may be electrically grounded for attracting the chargedparticulate emissions. It will be further appreciated that the coronamay be positively charged as well and, in this respect, any charge maybe used in accordance with the invention described herein. As such, theinvention is not intended to be limited only to the negative chargesdiscussed above.

FIGS. 6 and 7 show an alternate embodiment of the present invention, thevertical-flow wet vibrating precipitator in which the airflow starts ata horizontal flow and changes to a vertical flow. As shown in FIG. 6,separator 80 includes a duct 82 which includes a horizontal gas inlet 84and a vertical gas outlet 86. The duct 82 includes the horizontal ductportion 88, which leads to vertical duct portion 90. The ductincorporates a separation unit 92 which includes a plurality of cords 94similar to cords 24 in FIGS. 1 and 2. These are stretched at an inclinebetween a water inlet 96 and sidewall 98 of the duct 82. A watercollection trough 100 is located at the ends of cords 94 and directswater 102 which falls from the cords 94 down the vertical portion 90 ofduct 82 and across the horizontal portion 88 to a water collectiontrough 104.

A pump 106 directs the water from trough 104 through conduit 108 in thedirection arrows 110 back to the water source 96. As in the previousembodiments recirculated liquid may be first pass through a heatexchanger. Thus, in operation, as air is introduced into inlet 84, itwill pass through dropping water 102, which will cause the particles tocombine in droplets of water, forming larger particles. The air willthen pass upwardly through horizontal passage 90, through the sievesformed by cords 94. The water introduced into inlet 96 will run down andthrough cords 94 towards sidewall 98. As the water moves downwardly,along with the angled cords, it will drop into trough 100, whichincludes a plurality of holes 101, which allow the collected water todrip into trough 104 as shown by arrows 102. Again, in this embodiment,the cords 94 are designed to vibrate and create a trailing vortex. Thisis achieved by selecting the appropriate cord material, length andtension relative to the airflow through the sieve assembly 92.

Thus, this embodiment incorporates both a curtain of water shown byarrows 102 which facilitates the agglomeration of the smaller particlesinto larger particles while, at the same time, providing the vibratingsieve assembly 92 according to the present invention.

While the present invention has been illustrated by the description ofone or more embodiments thereof, and while the embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail. Thevarious features shown and described herein may be used alone or in anycombination.

What is claimed is:
 1. A sieve assembly for treating an exhaust,comprising; a plurality of sieves arranged relative to each other todefine a plurality of gaps therebetween for receiving the exhausttherethrough, each of the sieves including an elongated cordlongitudinally extending therealong and configured to transverselyvibrate between a first position and a second position such that each ofthe elongated cords generates a trailing vortex within the exhaust forcollecting a plurality of particulates within the trailing vortex. 2.The sieve assembly of claim 1, further comprising: a first supportmember and an opposing second support member, the plurality of sievesconnected to and extending between the first and second support memberssuch that each of the elongated cords is held in tension therebetween.3. The sieve assembly of claim 2, wherein each of the elongated cords isheld in a predetermined tension such that each of the elongated cords isconfigured to vibrate by directing a flow of the exhaust transverselytherealong.
 4. The sieve assembly of claim 1, further comprising: afirst support member defining a liquid supply conduit configured toreceive the liquid, the plurality of sieves projecting from the supportmember such that each of the elongated cords is fluidly connected to theliquid supply conduit to receive the liquid and pass liquid therealongas each of the elongated cords vibrates.
 5. The sieve assembly claimedin claim 4 further comprising a liquid collector connected to a heatexchanger.
 6. The sieve assembly of claim 4, wherein each of theelongated cords are formed from a hydrophobic material.
 7. The sieveassembly of claim 1, wherein each of the elongated cords is formed froma wire rope.